Finding the right nursing bra can be difficult, and it is very important. If you are not comfortable, nursing can be difficult, and the wrong size bra can lead to clogged milk ducts.
This actually happened to me so I am very well aware of the discomfort. Before my son was born I went and got measured for a new bra. the sales lady told me my size and recommended that I go up a size or two for when my milk came in. I purchased two bras, thinking I would order more online, from a cheaper retailer.
Well, I am so glad it worked out that way, because I actually went up three sizes! When my milk cam in the bra no longer fit. I was already uncomfortable from the milk engorgement those first few days and then on top of it my bra was too small. I ended up with a clogged duct. I stopped wearing those bras immediately and remeasured myself, and ordered the correct size online.
For starters I would purchase a nursing sports bra in a size bigger than usual and/or a Sleep/Transition bra. These will accommodate the changes you are going through just before delivery and shortly afterwards.. It has more give than a regular bra and so it should work until you can figure out your post milk size. Once your milk comes in, you will want to look for a bra that offers more support. It should be made from a breathable material and though it should be supportive, you should avoid underwire options. The tissue that produces milk extends all the way back to your rib cage and up into your armpit, therefore an underwire bra may obstruct the milk ducts leading to problems.
You should start with three bras: one to wear, one in the laundry, and a back up clean bra. Once you find a style and brand that is comfortable, purchase a few more.
Finding the right nursing bra takes time and effort, but it is well worth the search!